Fuel system for engines

ABSTRACT

A fuel system, particularly for a diesel engine, has a closed loop control system including an electronic governor powered by positive and negative supply lines and incorporating a summing amplifier the output from which controls the pump. The noninverting input of the amplifier is connected to a third supply line, which is maintained at a fixed percentage of the potential between the positive and negative supply lines. Also, the signals applied to the amplifier have magnitudes proportional to the potential between the positive and negative supply lines, so that fluctuations in the supply voltage do not affect the operation of the system.

United States Patent Williams et al. 1 Aug. 5, 1975 {54] FUEL SYSTEM FORENGINES 3.578.871 /1971 Sakumoto l23/32 EA 3.630177 12/197] Engel .1[23/32 EA [I'lVCntOl'SI Malcolm wllllams, SOllhUli'. 36659OU 5/ 97 l I HGeoffrey Albert Kenyon Brunt, 3.693.603 9/1972 Lemanczyk 123/32 EAGlaston ury; Chri ph r R bin 3.699.935 10/1972 Adler 123/32 EA Jones,Alccster. all of England 3.707.950 1/1973 Schlimme... l23/32 EA3.724.433 4/1973 V055 123/32 EA {731 Assgneci Bmmgham 3.777.174 12/1973ButSCher 123/32 EA England 22 Filed; Apr. 4 973 Primar E, :aminerCharlesJ. Myhre Assistant E.\'uminerRonald B. Cox I2] I Appl' Nu: 347'730Attorney, Agent. or FirmHolman 8L Stem Foreign Application Priority Datai 1 ABSTRACT Apr 4. 1972 United Kingdom 15340/72 A fuel System,Particularly for a diesel engine. has a W73 Uniwd Kingdom 15355; closedloop control system including an electronic governor powered by positiveand negative supply [52] us C| H 123/139 290740; 0 lines andincorporating a summing amplifier the out- 6059'09 put from whichcontrols the pump. The non-inverting 1. 11 int. c1. F02m 39/00; F02b3/00 input of the amplifier is connected to a third pp [53] Field 0Search 33 3 5 33 EA 32 AE line, which is maintained at a fixedpercentage of the 33 1 1 3935 393 2 40 potential between the positiveand negative supply lines. Also. the signals applied to the amplifierhave I 5 References Cited magnitudes proportional to the potentialbetween the UNITED STATES PATENTS positive and negative supply lines, sothat fluctuations in the supply voltage do not affect the operation ofthe 3.425.401 2/!969 Lang a a 1 123/32 EA System- 3.478.5l2 ll/l969Brahm 123/32 EA 3.543.792 l2/l970 Palmer 123/32 EA 4 Claims, 9 DrawingFigures l3 I A 2| SHEET PATENTED AUG 5 I975 U P T U 3 0 DI l. M 0 0 l W2 F v o 6 9 i b 2 w i 3 2 3 2r 2 a 7.. P. /3 01 D 2 8 II E l 1 6 v 2 s 2FIG. 4

3, PATENTEUAUG 5191s 897 763 sum 2 PUMP OUTPUT SPEED FIG? SHEET PATENTEDAUG 5 I975 FIG.8

FIG.9

FUEL SYSTEM FOR ENGINES This invention relates to fuel systems forengines.

A fuel system according to the invention includes a pump supplying fuelto the engine, and a closed loop control system controlling the pump,the closed loop control system including an electronic governor which ispowered by positive and negative supply lines and incorporates a summingamplifier the output from which controls the pump, said amplifier havingits noninverting input connected to a third supply line which ismaintained at a fixed percentage of the potential between the positiveand negative supply lines, and having applied to its inverting input atleast two signals of opposite polarity with respect to said third supplyline, said signals having magnitudes proportional to the potentialbetween the positive and negative supply lines.

In the accompanying drawings,

FIG. 1 is a circuit diagram, partly in block form, illustrating oneexample of the invention.

FIGS. 2 to 4 are graphs illustrating the outputs of three transducersused in FIG. 1.

FIG. 5 represents a fuel-speed characteristic for an engine to becontrolled by the arrangement of FIG. 1.

FIG. 6 is a view similar to FIG. 1 of a second example of the invention.

FIG. 7 is a view similar to FIG. 5 but showing the characteristicobtained by FIG. 6.

FIG. 8 is a circuit diagram showing one form of power supply circuit,and

FIG. 9 is a circuit diagram illustrating one form of check circuit forthe power supply circuit.

All the examples described relate to a fuel injection system for adiesel engine driving a road vehicle, so that demand is set by anaccelerator pedal. However, the arrangements shown can be used withother engines, and the engine employed need not drive a road vehicle, inwhich case the demand is of course set in some other way.

Referring first to FIG. 1, a fuel pump 11 supplies fuel to the cylindersof an engine 12 in turn, the fuel pump being driven in a conventionalmanner, with the timing of injection controlled in the usual way. Thedriving of the fuel pump forms no part of the present invention and isnot therefore described. Moreover, the type of pump used is notcritical, but in the example shown the pump is a conventional in-linepump having a control rod 14 the axial position of which determines therate of supply of fuel to the engine 12 by the pump 11. The axialposition of the control rod 14 is controlled by an electromechanicalactuator 13 to determine the pump output.

The system further includes three transducers 15, 16 and 17. Thetransducer produces an output in the form of a voltage shown in FIG. 2,the magnitude of the voltage being dependent on the rotational speed ofthe engine. The transducer 16 produces an output voltage shown in FIG. 3the voltage being dependent on the rate of supply of fuel to the engine,(i.e. the pump output). For this purpose the transducer 16 convenientlysenses the axial position of the control rod 14 as indicated by thedotted line. The transducer 17 produces a voltage representing demand.Typically, the transducer 17 is controlled by the accelerator pedal ofthe vehicle which is driven by the engine, and in the particular examplebeing described, the engine is controlled by an all-speed governor, sothat the output from the transducer 17 is a voltage representingdemanded engine speed. The form of this voltage is shown in FIG. 4, andit should be noted that the slope of this output is opposite to theslopes of the outputs from the transducers 15, 16.

The outputs from the transducers 15, 16 and 17 are all applied, by wayof resistors 15a, 16a, 17a, converting the signals to current signals,to the inverting terminal of an operational amplifier 18 connected as asumming amplifier, whilst the output from the transducer 16 is alsoconnected through a resistor 16b to the inverting terminal of anoperational amplifier 19 connected as a summing amplifier. Theamplifiers 18 and 19 are powered by positive and negative supply lines21, 22 derived from the vehicle battery and have their non-invertingterminals connected to a line 23 which is kept at a reference potentialmid-way between the potentials of the lines 21, 22. The origin in FIGS.2 to 4 is the potential of the line 23. All the transducers are poweredfrom the supply lines, which are of course intended to remain atconstant potentials. However, the transducer outputs are proportional tothe potential between the lines 21, 22 and so if this potential varies,so will the transducer outputs.

The output from the amplifier 18 is fed through a diode 24 to a drivecircuit 25 which incorporates a power amplifier and which serves tocontrol the electro-mechanical actuator 13. Similarly, the outputterminal of the amplifier 19 is connected to the drive circuit 25through a diode 26. The diodes 24 and 26 together constitute adiscriminator, which ensures that only the amplifier 18, 19 producingthe more positive output is coupled to the drive circuit 25 at any giveninstant. Thus, if the amplifier 18 is producing the more positiveoutput, then the diode 26 is reverse biased, and if the amplifier 19 isproducing the more positive output, the diode 24 is reverse biased. FIG.1 also shows the feedback resistors 27, 28 associated with theamplifiers 18, 19 respectively, and it will be noted that the feedbackcircuit for each amplifier is taken from the input terminal of the drivecircuit 25. By virtue of this arrangement, the effective forward voltagedrop across the diodes 24 and 26 is reduced by a factor dependent uponthe amplifier open-loop gain, and so the temperature characteristics ofthe diodes become negligible when considering the temperaturecharacteristics of the system. Also, there is a very sharp changeoverfrom control by one amplifier to control by the other amplifier.

The basic operation is as follows. The amplifier 18 receives currentinputs representing demanded speed, actual speed, and pump output. Theseinputs are compared, and the amplifier 18 produces an output which isfed to the drive circuit 25, and modifies the pump output until theinput signals to the amplifier 18 are balanced, at which point theoutput from the amplifier 18 becomes such that the drive circuit 25produces just sufficient current to keep the control rod 14 in theposition it has assumed.

There is an inversion stage between the amplifiers l8, l9 and the pump,so that the smaller the output from the amplifier 18, the greater thedemanded pump output. The amplifier 19 receives a signal by way of theresistor 16b representing pump output and also receives a referencecurrent from a reference source 20. If the demanded pump output exceedsa predetermined value, then the amplifier 19 produces a positive outputwhich is more positive than the output of the amplifier 18, so that thediode 24 ceases to conduct as previously explained and the amplifier 19produces an output to the drive circuit 25 to limit the pump output.When the amplifier 19 is producing an output, the system operates in thesame way as when the amplifier 18 is producing an output to reduce theoutput of the amplifier 19 to a value such that the output from thedrive circuit 25 keeps the control rod 14 in the position it hasassumed. The system will stay in this condition until the amplifier I8demands less fuel than the maximum set by the amplifier 19. When theamplifier l8 demands less fuel, it produces a greater positive outputthan the amplifier l9, and so takes over the operation.

Referring now to FIG. 5, the way in which the governor is designed andoperates can be seen from the graph of pump output against speed. Thisgraph also shows the effect of a number of controls not yet mentioned inrelation to FIG. 1. The line 40 is set by the amplifier 18 by virtue ofthe way in which the comparison of actual and demanded speeds ismodified in accordance with the input from the transducer 16. The line40 in the drawings represents 50 percent demand, and is one of a familyof curves stretching from demand to l00 percent demand. The extremes ofthis family, that is to say no demand and full demand, are indicated at38 and 43. The line 38 is set by a current source 31 providing an inputto the inverting terminal of the amplifier 18, to ensure that the enginespeed varies with pump output in the manner indicated by the line 38even when the demand is zero. The maximum speed is set by a control 29shown in FIG. 1 and which acts by limiting the maximum demand from thetransducer 17. The line 35 is the maximum fuel line which is set by theamplifier 19 as previously explained.

The boundary line 39 is a function of the engine, not the governor, andrepresents the no'load fuel requirements of the engine under differentdemands, so that the points 41 and 42 are the no-load engine speeds atzero and full demand, (i.e.) with the pedal released and fully depressedrespectively.

FIG. explains how the engine will behave in any circumstances. Supposethat the pedal has been set to demand 50 percent, corresponding to theline 40 shown in FIG. 5. The exact position on the line 40 at any giveninstant will depend upon the load on the engine, and so for this givensetting of the pedal, the engine speed can vary within the limits set bythe lines and 40. The slope of the line is, as previously explained, aresult of the input to the amplifier 18 from the transducer 16. Assumingthat the engine is operating at a particular point on the line 40, thenif the vehicle starts to go up an incline, the load will increase, andso for a given position of the pedal the operating point will move upthe line 40, so that the speed is reduced. If the load becomessufficiently great, the line 35 will be reached, and no further increasein pump output will be permit ted. At this point, the speed fallsrapidly. If the load decreases, then the operating point moves down theline 40 with the corresponding increase in speed. If the load decreasesto zero, the line 39 is reached.

[f the demand is changed, then assuming for the sake of argument that itchanges from percent demand to 100 percent demand, the pump output willincrease as rapidly as the pump and governor will allow until the line35 is reached, and the engine will then move along the line 35 onto themaximum demand line 43, and will assume a position on the line 43 whichis dependent upon the load.

If the demand is reduced, then assuming the demand is reduced from 50 to0 percent, the operating point will move downward until the fuel supplyis zero. The speed then decreases until the line 38 is reached, afterwhich the operating point moves up the line 38, finishing at a point onthe line 38 determined by the load on the engine.

Turning now to FIG. 6, there is shown a second example in which thegovernor is a two-speed governor, that is to say a governor in which thedemand signal is a fuel signal which is compared with the actual fuel,the pump output then being modified to provide the desired fuel output.In FIG. 6, the amplifier 18 receives a signal from the transducer 16 byway of the resistor 16a, this signal representing actual fuel. A signalrepresenting demanded fuel is fed by way of the resistor 17a to theamplifier 18, but it will be noted that there is no speed term set tothe amplifier 18 from the transducer 15. The characteristics of thesystem are shown in FIG. 7. The line 40a is one ofa family ofhorizontally extending lines which are set by the governor, and can betaken to represent the 50 percent demand line. When the pedal sets ademand of 50 percent, the amplifier 18 sets the required fuel level. Theoperating point on the line 400 will of course then depend on the loadon the engine.

The amplifier 19 overrides the amplifier 18 in FIG. 6 in a similarmanner to the arrangement in FIG. 1, except that the amplifier 19 nowreceives a signal by way of the resistor 15a representing speed, andalso a reference current from a source 200 indicating the maximum enginespeed. The amplifier 19 sets the maximum speed of the engine, which isindicated by the line 43 in FIG. 7. It will be noted that the line 43has a slope, that is to say the maximum permitted speed varies with pumpoutput. This slope is obtained by feeding to the amplifier 19 a signalrepresenting pump output, this signal being fed by way of the resistor16b.

The maximum pump output, that is to say the line 35 in FIG. 7, is set bya control 29a which limits the maximum demand, in much the same way asthe control 29 limits the maximum speed in FIG. 1. The minimum enginespeed, indicated by the line 38, is set by a current source 310, whichis similar to the current source 31 except that because the currentsource 310 acts on the amplifier 18, which does not receive a speedterm, the current source 310 must receive a speed term as indicated byits connection to the transducer 15.

It is extremely important that the line 23 is maintained at a potentialwhich is a fixed percentage of the potential between the lines 21, 22.As described above, this percentage is 50 percent. Because theamplifiers l8 and 19 have their non-inverting inputs connected to theline 23, the transducers have outputs which are proportional to thepotential between the lines 21, 22 and the overall system is a closedloop control system, then the fact that the line 23 is maintained at aconstant percentage of the potential between the lines 21, 22 ensuresthat variations in the supply voltage do not alter the operation of thesystem. This of course is of considerable importance where the system isemployed on a vehicle, and the basic power source is the battery of thevehicle, the voltage of which can vary.

In order to understand the effect of a variation in battery voltage,assume for example that the lines 21, 22

and 23 are at 20 volts, 0 volts and I0 volts respectively. If theresistors a, 16a, 17a are equal, then in steady state conditions, thetransducers 15, 16, 17 may produce output voltages of 13, 13 and 4respectively. It will be seen that the 4 volts, which is 6 volts belowthe reference, balances the two l3 volt outputs, each of which is 3volts above the reference. Suppose now, taking an extreme case by way ofillustration, that the line 21 falls to 10 volts. For fixed values ofthe parameters, the transducer output voltages become 6.5, 6.5 and 2respectively. However, the reference potential now becomes 5 volts, sothat the system is still in balance.

It should be noted that the various references employed are also derivedfrom the supply lines, and so are proportional to the potential betweenthe lines 21, 23.

Similar considerations of course apply to the amplifier 19, and to theamplifiers 18 and 19 in FIG. 6.

One arrangement for maintaining the line 23 at a fixed percentage of thepotential between the lines 21, 22 is shown in FIG. 8. The arrangementis powered by the battery 52 of a road vehicle, the battery 52 havingits negative terminal connected to the line 22, and its positiveterminal connected to the collector of an n-p-n transistor 56, theemitter of which is connected to the line 21. The collector and base ofthe transistor 56 are connected respectively to the collector andemitter of a further n-p-n transistor 57, the base of which is connectedto the positive battery terminal through a resistor 59, and to theterminal 22 through a Zener diode 58.

There is further provided a potentiometer chain comprising resistors 60and 61 connected in series between the lines 21, 22. A pointintermediate the resistor 60, 6| is connected to the base of an n-p-ntransistor 62 the collector of which is connected to the line 21 and theemitter of which is connected to the line 22 by way of a resistor 63. Inaddition, a further n-p-n transistor 64 is provided having its emitterconnected to the emitter of the transistor 62 and its collectorconnected to the terminal 21 by way of a resistor 65. The base of thetransistor 64 is connected to the line 23.

Connected between the lines 21, 23 is a resistor 66 and in parallel withthe resistor 66 is a series combination of a resistor 67 and thecollector-emitter path of an n-p-n transistor 68. The base of thetransistor 68 is connected to the emitter ofa further n-p-n transistor69 and the base of the transistor 69 is connected to the collector ofthe transistor 64. The collector of the transistor 69 is connected tothe terminal 21, and the emitter of the transistor 69 is connected byway of a resistor 70 to the line 23.

In operation, the transistors 56 and 57 together with the Zener diode 58form a voltage stabilizing network so that a substantially constantvoltage is obtained between the lines 21, 22. Also shown in FIG. 8 aretwo loads 53, 54 which indicate current consuming portions of thegovernors. In the particular example, the load 54 consumes more currentthan the load 53 and the resistor 66 supplies the majority of thesurplus current for the load 54. In order to maintain the voltage at theline 23 at substantially the desired value the conduction of thetransistor 68 is varied, and the transistor 68 together with theresistor 67 acts as a variable resistor in parallel with the resistor66. The resistors 60 and 61 are of equal value, so that the base voltageof the transistor 62 is maintained at the desired value of the voltageat the line 23 and the transistors 62 and 64 act as a differentialamplifier to control the base current of the transistors 69, 68 inaccordance with the difference between the desired value and the actualvalue of the voltage at the line 23. With this arrangement should thevalue of the voltage at the line 23 tend to vary in either directionthen the current flowing through the transistor 68 is varied accordinglyto maintain the voltage substantially constant as a known fraction ofthe voltage between the lines 21, 22. This ratio will be maintained asthe supply voltage varies, even if the supply voltage should fall belowthe breakdown voltage of the Zener diode 18, for example when crankingthe engine.

FIG. 9 shows an arrangement for checking the potentials of the lines 21,22, 23, and effecting a control when necessary. For this purpose, afirst check circuit is provided which comprises an n-p-n transistor 83the base of which is connected to a potential divider chain constitutedby resistors 84 and 85 connected in series between the lines 21, 22. Theemitter of the transistor 83 is connected to the line 23. The collectorof the transistor is connected to a first safety circuit 86 which isassociated with the power amplifier 25. The safety circuit 86 serveswhen operated to disable the power amplifier 25 so that no current issupplied to the actuator 13. The arrangement is such that in the eventthat the voltage at the line 23 should fall below 50 percent of thevoltage at the line 21, then the transistor 83 will conduct to causeoperation of the circuit 86. In the event that the voltage at the line23 reverts to its correct value then the safety circuit 86 will cease tooperate and the amplifier 25 operates again. Thus, the circuit checksthat the voltage at the line 23 relative has not fallen below apredetermined percentage of the voltage between the lines 21, 22.

It is also necessary to sense when the voltage at the line 23 risesabove a predetermined value. This is achieved by sensing the voltage atthe line 23 relative to the line 21 and comparing it with a standardvoltage. It will be understood of course that the first check circuitonly checks for a low voltage at the line 23 relative to the desiredproportion of the voltage between the lines 21, 22. The second checkcircuit comprises a p-n-p transistor 87 the emitter of which isconnected by way of a diode 88 to the line 22. The base of thetransistor 87 is connected through a resistor 89 to a point intermediatea resistor 90 and a Zener diode 91 connected in series between the lines21, 22. Thus the voltage on the base of the transistor 87 will bedetermined by the breakdown voltage of the Zener diode 91. The collectorof the transistor 87 is connected to a second safety circuit 92associated with the power amplifier 25. The safety circuit 92 in thisexample serves to prevent the supply of current by the power amplifier25 to the actuator 13. In operation, in the event that the voltage atthe line 23 rises above the predetermined value set by the Zener diode,the circuit 92 will be operated to prevent the amplifier 25 supplyingcurrent to the actuator 13.

It will be noted that in the event that the voltage at the line 21 risesthen the voltage at the line 23 will also rise. Hence the second checkcircuit indirectly but effectively, checks that the voltage at the line21 has not risen above a desired value.

We claim:

1. A fuel system for an engine, comprising in combination, a pump forsupplying fuel to the engine, an actuator for controlling the amount offuel supplied to the engine, a power amplifier for supplying power tothe actuator, a control circuit for controlling the amount of powersupplied to the actuator by the power amplifier, means for supplying tothe control circuit signals indicative of the actual speed of theengine, the pump output and a manually determined demand signal, a powersupply circuit for connection to a source of supply, said power supplycircuit having three output lines and incorporating means maintainingthe voltage of one of the lines relative to one of the other lines at apredetermined percentage of the voltage between the other lines, saidpower supply circuit providing power for the control circuit, a firstcheck circuit for checking the voltage on said one line relative to thevoltages on said other lines, and a second check circuit for checkingthe actual voltage on said one line relative to one of the other lines,said check circuits when operated serving to prevent operation of thepower amplifier thereby cutting off the supply of power to the actuator.

2. A fuel system for an engine including a pump supplying fuel to theengine, and a closed loop control system controlling the pump, theclosed loop control system including an electronic governor which ispowered by positive and negative supply lines and incorporates a summingamplifier the output from which controls the pump, said amplifier havingits non-inverting input connected to a third supply line and havingapplied to its inverting input at least two signals of opposite polaritywith respect to said third supply line, said signals having magnitudesproportional to the potential between said positive and negative supplylines, a battery providing power for the system, a voltage regulatorcoupling the battery to the positive and negative supply lines, variableresistance means coupling the third supply line to one of said supplylines, and comparator means continuously monitoring the potential on thethird supply line and varying said resistance means to maintain thepotential on said third supply line at a fixed percentage of thepotential between said positive and negative supply lines.

3. A fuel system for an engine including a pump supplying fuel to theengine, and a closed loop control system controlling the pump, theclosed loop control system including an electronic governor which ispowered by positive and negative supply lines and incorporates a summingamplifier the output from which controls the pump, said amplifier havingits non-inverting input connected to a third supply line and havingapplied to its inverting input at least two signals of opposite polarity with respect to said third supply line, said signals havingmagnitudes proportional to the potential between said positive andnegative supply lines, means monitoring the potentials of the positiveand negative supply lines and maintaining said third supply line at afixed percentage of the potential between the positive and negativesupply lines, a check circuit for effecting a control to prevent supplyof fuel to the engine if the potential of the third supply rail fallsbelow said fixed percentage, the check circuit including a transistorwith emitter and base terminals, one of said terminals being connectedto the third supply line and the other of said terminals being connectedto a potential divider connected across the positive and negative supplylines, the transistor turning on to effect the required control.

4. A fuel system for an engine including a pump for supplying fuel tothe engine, and a closed loop control system controlling the pump, theclosed loop control system including an electronic governor which ispowered by positive and negative supply lines and incorporates a summingamplifier the output from which controls the pump, said amplifier havingits non-inverting input connected to a third supply line and havingapplied to its inverting input at least two signals of opposite polaritywith respect to said third supply line, said signals having magnitudesproportional to the potential between said positive and negative supplylines, means monitoring the potentials of the positive and negativesupply lines and maintaining said third supply line at a fixedpercentage of the potential between the positive and negative supplylines, a check circuit for effecting a control to prevent supply of fuelto the engine if the potential on the third supply line rises to apredetermined value, the check circuit including a transistor withemitter and base terminals, one of said terminals being maintained at afixed potential by means of a zener diode and the other terminal beingconnected to the third supply line, the transistor turning on to effectthe required control.

l t i

1. A fuel system for an engine, comprising in combination, a pump forsupplying fuel to the engine, an actuator for controlling the amount offuel supplied to the engine, a power amplifier for supplying power tothe actuator, a control circuit for controlling the amount of powersupplied to the actuator by the power amplifier, means for supplying tothe control circuit signals indicative of the actual speed of theengine, the pump output and a manually determined demand signal, a powersupply circuit for connection to a source of supply, said power supplycircuit having three output lines and incorporating means maintainingthe voltage of one of the lines relative to one of the other lines at apredetermined percentage of the voltage between the other lines, saidpower supply circuit providing power for the control circuit, a firstcheck circuit for checking the voltage on said one line relative to thevoltages on said other lines, and a second check circuit for checkingthe actual voltage on said one line relative to one of the other lines,said check circuits when operated serving to prevent operation of thepower amplifier thereby cutting oFf the supply of power to the actuator.2. A fuel system for an engine including a pump supplying fuel to theengine, and a closed loop control system controlling the pump, theclosed loop control system including an electronic governor which ispowered by positive and negative supply lines and incorporates a summingamplifier the output from which controls the pump, said amplifier havingits non-inverting input connected to a third supply line and havingapplied to its inverting input at least two signals of opposite polaritywith respect to said third supply line, said signals having magnitudesproportional to the potential between said positive and negative supplylines, a battery providing power for the system, a voltage regulatorcoupling the battery to the positive and negative supply lines, variableresistance means coupling the third supply line to one of said supplylines, and comparator means continuously monitoring the potential on thethird supply line and varying said resistance means to maintain thepotential on said third supply line at a fixed percentage of thepotential between said positive and negative supply lines.
 3. A fuelsystem for an engine including a pump supplying fuel to the engine, anda closed loop control system controlling the pump, the closed loopcontrol system including an electronic governor which is powered bypositive and negative supply lines and incorporates a summing amplifierthe output from which controls the pump, said amplifier having itsnon-inverting input connected to a third supply line and having appliedto its inverting input at least two signals of opposite polarity withrespect to said third supply line, said signals having magnitudesproportional to the potential between said positive and negative supplylines, means monitoring the potentials of the positive and negativesupply lines and maintaining said third supply line at a fixedpercentage of the potential between the positive and negative supplylines, a check circuit for effecting a control to prevent supply of fuelto the engine if the potential of the third supply rail falls below saidfixed percentage, the check circuit including a transistor with emitterand base terminals, one of said terminals being connected to the thirdsupply line and the other of said terminals being connected to apotential divider connected across the positive and negative supplylines, the transistor turning on to effect the required control.
 4. Afuel system for an engine including a pump for supplying fuel to theengine, and a closed loop control system controlling the pump, theclosed loop control system including an electronic governor which ispowered by positive and negative supply lines and incorporates a summingamplifier the output from which controls the pump, said amplifier havingits non-inverting input connected to a third supply line and havingapplied to its inverting input at least two signals of opposite polaritywith respect to said third supply line, said signals having magnitudesproportional to the potential between said positive and negative supplylines, means monitoring the potentials of the positive and negativesupply lines and maintaining said third supply line at a fixedpercentage of the potential between the positive and negative supplylines, a check circuit for effecting a control to prevent supply of fuelto the engine if the potential on the third supply line rises to apredetermined value, the check circuit including a transistor withemitter and base terminals, one of said terminals being maintained at afixed potential by means of a zener diode and the other terminal beingconnected to the third supply line, the transistor turning on to effectthe required control.